A means has been devised of estimating numerically the relative virulence of S. aureus strains, from computer stored phage typing patterns plus the categories of infections caused. A high ratio of serious infections to simple "colonization" results in a high "Index of Infections Potential". Analysis of over 20,000 phage typed S. aureus strains vis a vis infections caused has revealed that, as the phage pattern grows shorter (in the number of typing phages lysing the strain) the potential of the strain for causing infections becomes greater. Non-typable strains, long regarded as having low virulence, actually have more virulence than most strains.